In a conventional electric vehicle or hybrid vehicle, a power supply system supplies electric power to a main motor, which drives a vehicle. For example, in a power supply system according to JP 4120310, a main motor voltage converter is provided to output a voltage to a drive circuit of a main motor, and a system relay is provided between a DC power source and the main motor voltage converter. An auxiliary machine load drive circuit is provided between the system relay and the main motor voltage converter in parallel to the main motor voltage converter relative to the DC power source. Thus the power of the DC power source is divided and supplied to the main motor voltage converter and the auxiliary load drive circuit through the common system relay. The auxiliary machine load drive circuit is exemplified as a drive circuit for a power steering apparatus or a braking apparatus.
It is possible to reduce the number of batteries by sharing the DC power source for driving the main motor without providing a battery exclusive to a drive circuit of each auxiliary machine drive circuit. The voltage of the main motor drive battery is higher than that of the battery provided exclusively to the auxiliary machine. By using a higher voltage by the auxiliary machine drive circuit, a current required to provide the same power is reduced. As a result, current loss, which is proportional to the square of a current, is reduced and efficiency of power is improved.
According to JP 4120310, when the system relay turns off to shut off power supply upon failure or the like of the main motor or the main motor drive circuit, power supply to the auxiliary machine load is also simultaneously shut off and the auxiliary machine is disabled to operate. If a power steering apparatus or a brake apparatus, which relates to a function of turning and stopping of a vehicle, is disabled to operate, the steering assist operation and the braking operation of the vehicle are disabled. If a hold signal, which continues to hold the system relay in the on-state under a predetermined condition, is generated to ensure the continued operation of the auxiliary machine, the control becomes complicated and control calculation load increases.